Interpretive Summary: The greenbug aphid (Schizaphis graminum) was introduced into the US in the late 1880s and afterword became a serious pest on wheat, oat and barley. Although sorghum was reported as a host, it was not until 1968 that greenbug became a serious pest on that crop. The most effective control method is the planting of resistant varieties; however, the occurrence of greenbug biotypes has hampered the development and use of plant resistance as a management technique. Until the 1990s, the evolutionary status of greenbug biotypes was obscure. Molecular genetics approaches, especially mtDNA sequencing, suggested that greenbugs were host-adapted races whose evolution pre-dated the advent of modern agriculture by thousands of years. The simplistic model of biotype selection by plant resistance genes in a manner analogous to insecticide resistance was shown to be flawed. To further elucidate the evolutionary and taxonomic status of the greenbug and its biotypes, two nuclear genes were sequenced. The DNA sequences of two nuclear genes matched the evolutionary history as suggested by mtDNA sequences. The greenbug is a genetically diverse species comprised of two to three host-adapted races or sub-species. These host races live together in the same environments, but do not interbreed. The outbreak on sorghum during 1968 was the result of the introduction of a host race adapted to sorghum, and not selection by host resistance genes in crops. The molecular genetics data demonstrate that biotypes arose by genetic recombination during sexual reproduction within host-races.

Technical Abstract:
The greenbug aphid, Schizaphis graminum (Rondani) was introduced into the US in the late 1880s and it established quickly as a pest on wheat, oat and barley. Sorghum was also a host, but it was not until 1968 that greenbug became a serious pest on it. The most effective control method is the planting of resistant varieties; however, the occurrence of greenbug biotypes has hampered the development and use of plant resistance as a management technique. Until the 1990s, the evolutionary status of greenbug biotypes was obscure. Four mtDNA cytochrome oxidase subunit I (COI) haplotypes were previously identified, suggesting that S. graminum sensu lato was comprised of host-adapted races. To elucidate the current evolutionary and taxonomic status of the greenbug and its biotypes, two nuclear genes and introns were sequenced; cytochrome c (CytC) and elongation factor 1-alpha (EF1-alpha). Phylogenetic analysis of CytC sequences were in complete agreement with COI sequences and demonstrated three distinct evolutionary lineages in S. graminum. EF1-alpha DNA sequences were in partial agreement with COI and CytC sequences, and demonstrated two distinct evolutionary lineages. Host-adapted races in greenbug are sympatric and appear reproductively isolated. Agricultural biotypes in S. graminum likely arose by genetic recombination via meiosis during sexual reproduction within host-races. The 1968 greenbug outbreak on sorghum was the result of the introduction of a host race adapted to sorghum, and not selection by host resistance genes in crops.